In recent years, an imaging device is incorporated also in a portable terminal and a personal computer, etc. For an imaging optical system used for an imaging device such as a portable terminal (refer to Patent document 1, for example), a high image formation performance is required in association with fining of a pixel of an image pickup element. In addition, for such an imaging optical system, it is also required to achieve a smaller size in association with making a portable terminal, etc. thin. For this reason, a method for enhancing an image formation performance by making an imaging optical system smaller has been requested conventionally.